1. Field of the Invention
This invention relates to methods and apparatus for decreasing the concentration of hydroxyl ions in an alkaline hydrogen peroxide aqueous solution.
2. Description of the Prior Art
Peroxides are used in the bleaching of both high-yield and chemical pulps. When used under relatively mild conditions of 35.degree.-55.degree. C., peroxide is an effective lignin-preserving bleaching agent, improving the brightness of groundwood and other highly lignified pulps without significant yield loss. The pulp and paper industry commonly requires that peroxide bleach liquor compositions consist of one mole of total base to one mole of perhydroxyl ion, or a 1:1 mole ratio. This ratio is most usually expressed in the pulp and paper industry as a weight ratio of total alkalinity (expressed as sodium hydroxide) to perhydroxyl ion (expressed as hydrogen peroxide). Therefore, a 1:1 mole ratio would correspond to a 1.18:1 weight ratio.
Aqueous alkaline solutions of perhydroxyl ion can be generated electrochemically, for example, by the method of U.S. Pat. No. 4,693,794 or by electrochemical cells as disclosed in U.S. Pat. No. 4,406,758 which can utilize electrodes as described in U.S. Pat. No. 4,481,303. Other patents disclosing electrolytic cells for producing hydrogen peroxide are U.S. 4,758,317; 4,384,931; and 4,357,217. All these methods of producing an aqueous alkaline solution of a perhydroxyl ion involve a two electron reduction of oxygen at the cell cathode to effect the production of perhydroxyl (i.e., hydroperoxide) ions and hydroxyl ions according to the reaction: EQU O.sub.2 +H.sub.2 O+2e.sup.- .fwdarw.HO.sup.-.sub.2 +OH.sup.-
The electrolyte utilized in such electrochemical cells can be an aqueous alkali metal halide or preferably an aqueous alkali metal hydroxide. The electrolyte can be either supplied directly to the catholyte of the cell together with oxygen or through controlled flow across a cell separator. The flow is controlled by a net hydraulic pressure applied to the cell anode face of the cell separator. As can be seen from the above reaction, one mole of hydroxyl ion is produced with every mole of perhydroxyl ion. In the absence of any anion selectivity of the cell membrane or separator, and neglecting any decomposition of the peroxide, electrolytic losses of perhydroxyl ion or hydroxyl ion, the best molar ratio of hydroxyl ion to perhydroxyl ion is about 1:1. If, for example, an aqueous solution of sodium hydroxide is used as the electrolyte, an equimolar aqueous solution of sodium hydroxide and sodium hydroperoxide is obtained in the electrolytic cell. Since sodium hydroperoxide is a base, it contributes to the total alkalinity of the solution mixture. Therefore, a mixture of 1 mole of sodium hydroxide and 1 mole of sodium hydroperoxide produces a solution containing 2 moles of base when the total alkalinity is expressed as sodium hydroxide alone. Thus, the best molar ratio of total base to hydrogen peroxide which is obtainable utilizing an electrolytic cell to produce hydrogen peroxide is a molar ratio of about 2:1.
A 2:1 mole ratio of sodium hydroxide to hydrogen peroxide produced by an electrochemical cell process corresponds to a weight ratio of about 2.35:1 of sodium hydroxide to hydrogen peroxide. Bleaching solutions having this ratio of base to hydrogen peroxide find some use in the pulp and paper industry. By far the majority of the applications for hydrogen peroxide in the pulp and paper industry require much lower weight ratios, namely, about 0.8 to about 1.1:1 of base to hydrogen peroxide. In order for an electrolytic cell process for the production of hydrogen peroxide to satisfy the requirements of the pulp and paper industry, a method of removing excess sodium hydroxide from the electrolytic cell product would be desirable.
In Japanese patent disclosure 61-284591, Dec. 15, 1986 entitled "Production Method for Hydrogen Peroxide", there is disclosed that hydrogen peroxide can be separated by dialysis from an aqueous solution of an alkali metal hydroxide using a monovalent anion selective exchange membrane. No details of the anion exchange membrane or other details of the dialysis process for separation of the alkali metal hydroxide are disclosed in this reference.
The solvated ion diameter of the perhydroxyl ion has been suggested, by Appleby and Marie in "Electrochimica Acta", 24, 195-202, to be in the range of 6 Angstroms, or 6.times.10.sup.-4 microns. The diameter of a solvated hydroxyl ion, accordingly, must be somewhat less, if indeed, it behaves as a solvated ion in the absence of a potential gradient. The difference between the sizes of the perhydroxyl and the hydroxyl ion would appear to be too small to allow separation from each other efficiently by the class of membranes termed microporous. That is, no separation would occur unless some separation driving force other than a concentration gradient serves to modify the ion transport rate through the membrane to increase the rate over that which would be achieved by diffusion of the ions alone.
Ion exchange membranes exist which separate ion species on the basis of differences in the amount of charge, for instance, monovalent hydroxyl ions can be separated from divalent sulfate ions. Since the perhydroxyl and the hydroxyl ion are both monovalent, such membranes which discriminate between ions on the basis of charge would be of little value in separating the perhydroxyl ion from the hydroxyl ion. Other known ion exchange membranes, as exemplified by the permselective membranes sold under the tradename NAFION.RTM. by DuPont or under the tradename FLEMION by Asahi are characterized as strong ion exchange membranes. These membranes were developed for chloralkali electrolytic cells. They are characterized as being ion permeable instead of liquid or hydraulically permeable. Being strong ion exchange membranes, these membranes were developed to permit positive or negative ion transport while blocking the transport of oppositely charged ions. Since the perhydroxyl ion and the hydroxyl ion have the same negative charge, these membranes would not discriminate between these two negatively charged ions so as to permit one ion to pass through the membrane, yet block the other ion from passage. Therefore, no separation of hydroxyl ion from perhydroxyl ion would be expected to occur utilizing these prior art ion exchange membranes.
Another type of permselective membrane is characterized as a weak ion exchange membrane. One such membrane is described in D'Agostino et al U.S. Pat. No. 4,230,549 assigned to RAI Research Corporation, entitled "Separator Membranes for Electrochemical Cells". This type of weak ion exchange permselective membrane is also described by RAI Research Corporation as a base diffusion dialysis membrane capable of separating a base from pulp and paper industry effluents. In Chiang U.S. Pat. No. 4,731,173 entitled "Article for Constructing an Electrolytic Cell", the use of a weak permselective membrane is described for use in an electrolytic cell for the manufacture of hydrogen peroxide. This membrane is described as a low density polyethylene base film grafted to a weak base cation monomer. It is further described in Chiang as having a small mean pore size making it permeable to ions and not molecules, but having openings of sufficient size to permit the passage of gas bubbles without permitting substantial diffusion or back mixing of hydrogen peroxide from one compartment of the electrolytic cell to another.
There is no suggestion in any of these prior art references of a process to separate an hydroxyl ion from a perhydroxyl ion so as to reduce the alkalinity of the alkaline hydrogen peroxide product of an electrolytic cell.